Exhaust gas purification system and method of purifying exhaust gas

ABSTRACT

Decreases in the volume of intake air to an engine in response to environmental changes results in an increase in amount of PM emissions. In view of this situation, a correction coefficient of fuel supply interval is calculated based on the variation in amount of PM emissions to adjust a reference fuel supply interval in order to determine an target fuel supply interval. By adjusting the fuel supply interval, a fuel supply amount appropriate to the variation in amount of PM emissions, thereby preventing clogging of the injection hole of a supplemental fuel valve, while maintaining fuel economy.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2006-149654 filed on May 30, 2006 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system and a method for purifying exhaust gas from an internal combustion engine using a catalyst. More particularly, the invention relates to an exhaust gas purification system having a supplemental fuel valve that supplies fuel to an exhaust passage as well as to an exhaust gas purification method for supplying fuel to the exhaust passage.

2. Description of the Related Art

Generally, lean burn internal combustion engines, such as diesel engines, operate predominantly in lean burn mode with a high air-fuel ratio (lean mixture). Thus, such engines generally are equipped with a NO_(X) storage catalyst in the exhaust passage to purify the exhaust gas by absorbing nitrogen oxides (hereinafter “NO_(X)”) contained in exhaust gas.

When the amount of NOx absorbed by in the NO_(X) storage catalyst reaches saturation, an NOx reduction reaction is necessary to revive the NOx storing capacity of the catalyst. One approach to reducing NOx is to add an NOx reductant (light oil or other fuel) upstream of the NOx storage catalyst in the exhaust passage in order to decrease the oxygen concentration in the catalytic converter, and then, to use reductants, such as excess hydrocarbon and carbon monoxide, to promote NOx reduction.

Exhaust gas from diesel engines contains particulate matter whose principal component is carbon (hereinafter “PM”), soot, soluble organic fraction (SOF) and so forth. These emissions cause air pollution. A conventional exhaust gas purification system for diesel engines, which is designed to purify such PM and other emissions, has a particulate filter disposed in the exhaust passage. The particulate filter traps PM contained in the exhaust gas passing through the exhaust passage, thereby reducing the amount of PM emissions released to the atmosphere. For example, a diesel particulate filter (DPF) or a diesel particulate-NO_(X) reduction system (DPNR) catalyst may be used as a particulate filter.

PM deposits accumulate on the particulate filter as the amount of PM trapped in the filter increases, causing the particulate filter to become clogged with the PM deposits. Thus, pressure loss of the exhaust gas passing through the particulate filter increases, and accordingly, engine exhaust backpressure increases. This reduces engine power output and fuel economy. In order to solve the aforementioned problems, a conventional art supplies fuel to the exhaust passage (upstream of the particulate filter) to increase the exhaust gas temperature, thereby promoting oxidization (combustion) of the PM deposits on the particulate filter (PM catalyst regeneration process.)

As described above, in the NOx reduction process and the PM catalyst regeneration process, both intended to maintain the exhaust gas purification performance of the catalyst, fuel is supplied to the exhaust passage using a supplemental fuel valve disposed in the exhaust passage. However, because the injection hole of the supplemental fuel valve is exposed to the interior of the exhaust passage, some substances contained in the exhaust gas, such as soot and SOF, may adhere to and form deposits on the hole of the valve. This creates a concern that exposing the deposits to high-temperature exhaust gas may alter the properties of the substances and solidify them, and clog the hole of the valve. One example approach for preventing the supplemental fuel valve from becoming clogged is described in JP-A-2003-222019, in which fuel is supplied at all times, except during NOx reduction and PM catalyst regeneration, to decrease the temperature of the distal end of the supplemental fuel valve.

If the volume of intake air to the engine is decreased, due, for example, to environmental changes, such as changes in atmospheric pressure when driving from a lower altitude to higher altitude, or upon a shift from normal to transient driving. This results in an increase in amount of PM emissions. As the amount of PM emissions increases, the amount of PM adhering and entering the injection hole of the supplemental fuel valve increases, which helps PM deposits build up. The PM deposits may clog the injection hole of the supplemental fuel valve.

In order to solve such a problem as injection hole clogging, the supplemental amount of fuel (the supplemental amount of fuel per unit time) may be adjusted when the amount of PM emissions reaches the maximum in the allowable fluctuation. However, adjustment of the supplemental amount of fuel in such a manner may create another concern about a tendency of reduced fuel economy.

SUMMARY OF THE INVENTION

The invention provides an exhaust gas purification system that maintains fuel economy, while preventing clogging of the injection hole of the supplemental fuel valve.

A first aspect of the invention is directed to an exhaust gas purification system having a catalyst disposed in an exhaust passage in an internal combustion engine and a supplemental fuel valve for supplying fuel to the exhaust passage. The exhaust gas purification system includes an adjusting means for adjusting the amount of fuel that is supplied from the supplemental fuel valve to the exhaust passage based on the variation in the amount of particulate matter emissions from a combustion chamber in the internal combustion engine.

A second aspect of the invention is similar to the first aspect, except that the exhaust gas purification system further includes an adjusting means that adjusts the amount of fuel that is supplied from the supplemental fuel valve to the exhaust passage per unit time based on the variation in the amount of particulate matter (PM) emissions from the combustion chamber in the internal combustion engine.

The volume of intake air to the engine decreases upon environmental changes, such as atmospheric pressure change due to driving from a lower altitude to a higher altitude, or upon a shift from normal to transient driving. This results in an increase in amount of PM emissions. In view of such situation, the fuel supply amount per unit time is adjusted based on the variation in amount of PM emissions (e.g. variation in actual intake air volume). This provides a fuel supply amount appropriate to the variation in amount of PM emissions, thereby preventing excessive fuel supply. Therefore, clogging of the injection hole of the supplemental fuel valve is prevented, while fuel economy is maintained.

According to the second aspect, one approach to adjusting the fuel supply amount per unit time, in which a reference fuel supply interval is multiplied by a correction coefficient to determine an target fuel supply interval; where the reference fuel supply interval depends on the operating condition of the internal combustion engine, and the correction coefficient depends on the variation in amount of PM emissions. More specifically, in this approach, if the actual intake air volume to the internal combustion engine is smaller than the reference intake air volume (if the air volume ratio (actual intake air volume divided by reference intake air volume) is low), the reference fuel supply interval is multiplied by a correction coefficient to determine the target fuel supply interval, where the correction coefficient changes or shortens the fuel supply interval, that is, increasing the fuel supply amount per unit time.

In the exhaust gas purification system having the supplemental fuel valve for supplying fuel to the exhaust passage, as the atmospheric temperature (exhaust gas temperature) at the distal end of the supplemental fuel valve increases from a reference preset value, the temperature of the distal end of the supplemental fuel valve also increases, producing PM deposits. The PM deposits may clog the injection hole of the fuel supply valve. Therefore, there arises a need to increase the fuel supply amount. In view of this situation, according to a third aspect of the invention, the target fuel supply interval is determined in consideration of the temperature of the distal end of the supplemental fuel valve.

The third aspect of the invention is similar to the first aspect of the invention, except that exhaust gas purification system further includes: an adjusting means for adjusting the fuel supply interval for supplying fuel from the supplemental fuel valve to the exhaust passage based on the variation in amount of particulate matter emission from the combustion chamber in the internal combustion engine (variation in amount of PM emissions); and a supplemental fuel valve temperature estimating means for estimating a temperature of the supplemental fuel valve. The adjusting means compares first and second correction coefficients with each other, where the first correction coefficient changes or shortens the fuel supply interval if the actual intake air volume in the internal combustion engine is smaller than the reference intake air volume, and the second correction coefficient changes or shortens the fuel supply interval if the temperature of the supplemental fuel valve, estimated by the supplemental fuel valve temperature estimating means, increases. The reference fuel supply interval is multiplied the one of the first and second correction coefficients that results in a shorter fuel supply interval, to determine the target fuel supply interval.

As described above, the correction coefficient that results in a shorter fuel supply interval (a larger fuel supply amount per unit time) is selected to adjust the reference fuel supply interval, one correction coefficient depending on the variation in amount of PM emissions, the other correction coefficient depending on the temperature of the distal end of the supplemental fuel valve. This allows the fuel supply interval to be adjusted for either one of the condition changes that is more likely to cause clogging of the injection hole of the supplemental fuel valve; where the condition changes are a rise in temperature of the distal end of the supplemental fuel valve and an increase in amount of PM emissions due to environmental changes or during transient driving conditions. Thereby, clogging of the injection hole of the supplemental fuel valve can be effectively prevented. Moreover, a fuel supply amount appropriate to the foregoing specific condition change is provided. This prevents excessive fuel supply. Therefore, while fuel economy is maintained, and clogging of the injection hole of the supplemental fuel valve is prevented.

Although increasing the fuel supply amount per unit time prevents clogging of the injection hole of the supplemental fuel valve, the fuel will also react with oxygen in the catalyst, which may cause the catalyst temperature to exceed a certain range of values (e.g. 750° C.). One approach is offered to avoid this situation, in which when a catalyst temperature is equal to or higher than a prescribed value, the fuel supply amount per unit time is reduced depending on the catalyst temperature (i.e. variation in catalyst temperature relative to the preset value). This approach helps avoid a problem of thermal degradation of the catalyst due to excessively high catalyst temperatures caused by the increased amount of fuel supply.

One example approach is offered to reduce the fuel supply amount per unit time, in which the target fuel supply interval, shorter than the reference fuel supply interval, (e.g. the corrected target fuel supply interval) is used to shorten the fuel supply duration per interval shown in FIG. 8. This approach not only ensures a shorter fuel supply interval, which can prevent clogging of the injection hole of the supplemental fuel valve, but also ensures a smaller total amount of fuel supply. Thus, while an excessive increase in catalyst temperature is prevented, clogging of the injection hole of the supplemental fuel valve is also prevented.

Another approach is offered to prevent an excessive rise in catalyst temperature, in which a restrictive correction or increase of the fuel supply amount per unit time is performed such that the catalyst temperature, estimated based on the exhaust gas temperature, does not exceed a prescribed value.

The inventions according to the second and third aspects may further include a catalyst temperature detecting means that detects a temperature of the catalyst. If the variation in catalyst temperature, detected by the temperature detecting means, is equal to or higher than a preset value, the adjusting means may reduce the fuel supply amount per unit time depending on the variation in catalyst temperature.

The inventions according to the second and third aspects may further include a coolant temperature detecting means for detecting the temperature of the coolant in the internal combustion engine. If the variation in coolant temperature, detected by the coolant temperature detecting means, is equal to or greater than a preset value, the adjusting means reduces the fuel supply amount per unit time depending on the variation in coolant temperature.

The internal combustion engine may be a diesel engine. The internal combustion engine may be mounted on a vehicle.

Further, a fourth aspect of the invention is directed to an exhaust gas purification method that supplies fuel to an exhaust passage in an internal combustion engine, the exhaust passage having a catalyst disposed therein. The exhaust gas purification method includes adjusting the amount of fuel that is supplied from the supplemental fuel valve to the exhaust passage based on the variation in amount of particulate matter emission from a combustion chamber in the internal combustion engine.

According to the fourth aspect, the amount of fuel may be a amount of fuel that is supplied from the supplemental fuel valve to the exhaust passage per unit time.

According to the fourth aspect, in order to adjust the degree of fuel, the reference fuel supply interval may be multiplied by the correction coefficient that depends on the variation in amount of particulate matter emission to determine the target fuel supply interval.

According to the fourth aspect, in order to adjust the degree of fuel, if the actual intake air volume to the internal combustion engine is smaller than the reference intake air volume, the reference fuel supply interval may be multiplied by a correction coefficient for adjusting or shortening the fuel supply interval to determine the target fuel supply interval.

According to the fourth aspect, the degree of fuel may be an interval of fuel that is supplied from the supplemental fuel valve to the exhaust passage.

The invention according to the fourth aspect further may include estimating the temperature of the supplemental fuel valve. In order to adjust the degree of fuel, the first and second correction coefficients are compared with each other, where the first correction coefficient is for changing or shortening the fuel supply interval if the actual intake air volume to the internal combustion engine is smaller than the reference intake air volume, and the second correction coefficient is for changing or shortening the fuel supply interval if the temperature of the supplemental fuel valve, which is estimated by the supplemental fuel valve temperature estimating means, rises. Then, the reference fuel supply interval is multiplied by either one of the first and second correction coefficients, which results in a shorter fuel supply interval, and the target fuel supply interval is determined.

The invention according to the fourth aspect may further include detecting the temperature of the catalyst temperature. In order to adjust the degree of fuel, when the catalyst temperature detected by the catalyst temperature detecting means is equal to higher than a preset value, the fuel supply amount per unit time is reduced depending on the catalyst temperature.

According to the fourth aspect, a fuel injection amount per unit time may be reduced by adjusting or shortening the fuel supply interval, while the fuel supply duration per interval is reduced.

A fifth aspect of the invention is directed to an exhaust gas purification system having: a catalyst disposed in an exhaust passage in an internal combustion engine; a supplemental fuel valve for supplying fuel to the exhaust passage; and an adjusting portion for adjusting a degree of fuel that is supplied from the supplemental fuel valve to the exhaust passage based on the variation in amount of particulate matter emission from a combustion chamber in the internal combustion engine.

A sixth aspect of the invention the exhaust gas purification method may further include adjusting the degree of fuel that is supplied from the supplemental fuel valve to the exhaust passage based on the variation in amount of particulate matter emission from the combustion chamber in the internal combustion engine.

According to the aforementioned aspects of the invention, in view of the amount of PM emissions that can increase from the amount under the flat and normal driving conditions, the fuel supply amount per unit time (fuel supply interval) is adjusted based on the variation in amount of PM emissions. This provides a fuel supply amount appropriate to the variation in amount of PM emissions, thereby preventing clogging of the injection hole of the supplemental fuel valve, while maintaining fuel economy.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a schematic diagram showing an example of a diesel engine equipped with an exhaust gas purification system according to the invention.

FIG. 2 is a block diagram of the configuration of a control system, including an ECU.

FIG. 3 is a flowchart showing an example of a correction process of fuel supply interval executed by the ECU.

FIG. 4 is a map for calculating a reference fuel supply interval, which is used in the correction process of fuel supply interval of FIG. 3.

FIG. 5 is a map illustrating a correction coefficient of fuel supply interval, which is used in the correction process of fuel supply interval of FIG. 3.

FIG. 6 is a map illustrating λ correction coefficient for amount of PM emissions, which is used in the correction process of fuel supply interval of FIG. 3.

FIG. 7 is a map illustrating a correction coefficient of fuel supply interval, which is used in the correction process of fuel supply interval of FIG. 3.

FIG. 8 illustrates a fuel supply interval and supply duration.

FIG. 9 is another example illustrating the map of correction coefficient of fuel supply interval, which varies depending on the temperature of the distal end of the supplemental fuel valve.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment of the invention will be described below with reference to the drawings. A general configuration of a diesel engine using a fuel supply apparatus of the invention is described with reference to FIG. 1.

In this embodiment, the diesel engine 1 (hereinafter referred to as “engine 1”) may be a common rail in-cylinder direct-injection four-cylinder engine. The engine 1 includes, as main components, a fuel supply system 2, combustion chambers 3, an intake system 6 and an exhaust system 7.

The fuel supply system 2 includes a fuel supply pump 21, a common rail 22, injectors (fuel injection valves) 23, a supplemental fuel valve 25, an engine fuel passage 26 and a fuel passage 27.

The fuel supply pump 21 draws fuel from the fuel tank and pressurizes the fuel to supply the pressurized fuel to the common rail 22 through the engine fuel passage 26. The common rail 22 functions as an accumulator for maintaining the pressure of fuel supplied from the fuel supply pump 21 at a prescribed level (accumulating the high-pressure fuel supplied from the fuel supply pump 21). The common rail 22 distributes the accumulated fuel to the injectors 23. Each injector 23 is an electromagnetically driven valve designed to open when a specified voltage is applied and spray fuel into the associated combustion chamber 3.

The fuel supply pump 21 is designed to supply part of the fuel drawn from the fuel tank to the supplemental fuel valve 25 through the fuel passage 27. The supplemental fuel valve 25 is an electromagnetically driven valve designed to open when a specified voltage is applied and supply fuel to the exhaust system 7 (from exhaust ports 71 to an exhaust manifold 72). An injection hole of the supplemental fuel valve 25 is exposed to the interior of the exhaust system 7.

The intake system 6 has an intake manifold 63 connected to intake ports formed in the cylinder head. An intake pipe 64, included in the intake passage, is connected to the intake manifold 63. An air cleaner 65, an airflow meter 32 and a throttle valve 62 are disposed in the intake passage in order from the upstream side. The airflow meter 32 is designed to output an electric signal that indicates the volume of airflow into the intake passage through the air cleaner 65.

The exhaust system 7 has an exhaust manifold 72 connected to the exhaust ports 71 formed on the cylinder head. Exhaust pipes 73 and 74, included in the exhaust passage, are connected to the exhaust manifold 72. A catalytic converter 4 is also disposed in the exhaust passage.

The catalytic converter 4 includes a NO_(X) storage reduction catalyst 4 a and a DPNR catalyst 4 b. The NO_(X) storage reduction catalyst 4 a is designed to absorb NO_(X) in the presence of a high oxygen concentration in exhaust gas when the oxygen concentration in the exhaust gas is high and to reduce NO_(X) to NO₂ or NO as emissions in the presence of a low oxygen concentration and a large amount of reduction component (unburnt component of fuel, such as HC) in exhaust gas when the oxygen concentration is low and an excess of reductant (e.g., unburned fuel, such as HC) in exhaust gas. The NO_(X) emissions in the form of NO₂ or NO react immediately with HC or CO contained in exhaust gas, so that the NO₂ or NO is reduced to N₂. The reduction of NO₂ or NO to N₂ causes HC or CO to be oxidized to H₂O or CO₂.

In one example, the DPNR catalyst 4 b employs a porous ceramic structure that contains the NO_(X) storage reduction catalyst. The PM in exhaust gas is trapped when passing through the porous wall. When the air-fuel ratio of the exhaust gas is lean, the NO_(X) storage reduction catalyst absorbs NO_(X) contained in exhaust gas. When the air fuel ratio is rich, the stored NO_(X) is reduced. The DPNR catalyst 4 b also oxidizes and burns the trapped PM.

The exhaust gas purification system includes the catalytic converter 4, the supplemental fuel valve 25, and the fuel passage 27 as well as an electronic control unit (ECU) 100. The ECU 100 controls the operation of the supplemental fuel valve 25.

The engine 1 has a turbocharger (compressor) 5. The turbocharger 5 includes a turbine shaft 5 a, a turbine wheel 5 b and a compressor impeller 5 c, the turbine wheel 5 b and the compressor impeller 5 c are connected to each other via the turbine shaft 5 a. The compressor impeller 5 c faces the interior of the intake pipe 64, while the turbine wheel 5 b exposes the interior of the exhaust pipe 73. The turbocharger 5 thus configured utilizes an exhaust flow (exhaust pressure) received by the turbine wheel 5 b to rotate the compressor impeller 5 c in order to forcibly induct air into the engine. In this embodiment, the turbocharger 5 is a variable nozzle turbocharger having a variable nozzle vane mechanism 5 d on the side of the turbine wheel 5 b. The boost pressure of the engine 1 may be regulated by controlling the opening degree of the variable nozzle vane mechanism 5 d.

The intake system 6 has an intercooler 61 provided on the intake pipe 64. The intercooler 61 is designed to cool intake air whose temperature has increased due to the forced induction by the turbocharger 5. The throttle valve 62 is also provided in the intake pipe 64 downstream of the intercooler 61. The throttle valve 62 is an electronically controlled valve whose opening varies continuously. The throttle valve 62 reduces the cross-section of the intake air passage under certain conditions to control (decrease) the volume of intake air.

The engine 1 has an exhaust gas recirculation (EGR) passage 8 that connects the intake system 6 and the exhaust system 7. The EGR passage 8 recirculates some exhaust gas to the intake system 6 as required and supply such exhaust gas back to the combustion chambers 3 to lower the combustion temperature. This decreases the amount of NO_(X) emissions. The EGR passage 8 has an EGR valve 81 and an EGR cooler 82 that cools exhaust gas passing (recirculating) through the EGR passage 8. The volume of EGR to be introduced from the exhaust system 7 to the intake system 6 (volume of exhaust gas to be recirculated) may be adjusted by controlling the opening degree of the EGR valve 81.

The sensors will now be described. The engine 1 has several types of sensors installed at specific locations thereof. The sensors output signals that indicate the environmental conditions of the specific locations as well as signals indicating the operating conditions of the engine 1.

For instance, the airflow meter 32, upstream of the throttle valve 62 in the intake system 6, outputs a signal that indicates the detected flow rate of the intake air (intake air volume). The intake temperature sensor 33, provided on the intake manifold 63, outputs a signal that indicates the detected temperature of the intake air. The intake pressure sensor 34, provided on the intake manifold 63, outputs a signal that indicates the detected pressure of the intake air. An A/F (air-fuel ratio) sensor 35, downstream of the catalytic converter 4 in the exhaust system 7, outputs a detection signal, which continuously varies depending on the oxygen concentration in exhaust gas. An exhaust gas temperature sensor 36, downstream of the catalytic converter 4 in the exhaust system 7, outputs a that indicates the detected exhaust gas temperature. A rail pressure sensor 37 outputs a signal that indicates the detected pressure of the fuel stored in the common rail 22. A fuel pressure sensor 38 outputs a signal that indicates the detected pressure of fuel flowing through the fuel passage 27 (fuel pressure).

The ECU will now be described. As shown in FIG. 2, the ECU 100 includes a CPU 101, a ROM 102, a RAM 103 and a backup RAM 104. The ROM 102 stores several control programs, maps to be used for executing these control programs, and other data. The CPU 101 executes various operations in accordance with the respective control programs and maps stored in the ROM 102. The results of the operations in the CPU 101 and data inputted from the respective sensors are temporarily stored in RAM 103. The backup RAM 104 is a nonvolatile memory for saving stored data upon power-off, such as the engine 1 stop.

The ROM 102, the CPU 101, the RAM 103 and the backup RAM 104 are connected to each other via a bus 107, while being connected to an input interface 105 and an output interface 106.

The input interface 105 connects to the airflow meter 32, the intake temperature sensor 33, the intake pressure sensor 34, the A/F sensor 35, the exhaust gas temperature sensor 36, the rail pressure sensor 37, and the fuel pressure sensor 38. In addition, the input interface 105 connects to a water temperature sensor 31, an atmospheric pressure sensor 39, an accelerator depression sensor 40 and a crankshaft position sensor 41. The water temperature 31 outputs a signal that indicates the detected coolant temperature in the engine 1. The atmospheric pressure sensor 39 detects the atmospheric pressure variable due to environmental conditions, including altitude. The accelerator depression sensor 40 outputs a signal that indicates the detected displacement of the accelerator pedal. The crankshaft position sensor 41 outputs a pulse when the output shaft (crankshaft) of the engine 1 rotates by a given angle. In turn, the output interface 106 connects to the injector 23, the supplemental fuel valve 25, the variable nozzle vane mechanism 5 d, the throttle valve 62, the EGR valve 81 and others.

The ECU 100 executes the respective controls in the engine 1 based on the outputs from the aforementioned sensors. The ECU 100 also executes PM catalyst regeneration control and a correction process of fuel supply interval, which will be described later.

Next, the PM catalyst regeneration control will be described. The ECU 100 first estimates the amount of PM deposits in the DPNR catalyst 4 b. One approach to estimating the amount of PM deposits is to use a map plotted with experimental data on the amount of PM adhesion that varies depending on the operating conditions of the engine 1 (e.g. exhaust gas temperature, fuel injection amount and engine speed). The amounts of PM adhesion read from the map are summed to obtain the amount of PM deposits. Another approach would be to estimate the amount of PM deposits based on the vehicle driving distance or driving duration. Still another alternative is to use a pressure differential sensor, disposed in the catalytic converter 4, to detect the pressure differential between upstream and downstream of the DPNR catalyst 4 b. The amount of PM deposits trapped by the DPNR catalyst 4 b is calculated based on the output from the differential pressure sensor.

If the estimate amount of PM deposits is equal to or larger than a specified reference value, the ECU 100 determines to start regeneration of the DPNR catalyst 4 b and executes the PM catalyst regeneration control. More specifically, the ECU 100 calculates a required fuel supply amount and supply interval based on the engine speed output from the crankshaft position sensor 41 with reference to the map previously plotted with the experimental results. According to the calculation result, the ECU 100 controls the operation of the supplemental fuel valve 25, through which fuel is supplied to the exhaust system 7 continuously. The fuel supply results in a rise in temperature of the DPNR catalyst 4 b, which promotes oxidization of the PM deposits in the DPNR catalyst 4 b to H₂O and CO₂ emissions.

Other than the PM catalyst regeneration control, the ECU 100 may execute sulfur poisoning recovery control or NO_(X) reduction control. The sulfur poisoning recovery control releases sulfur from the NO_(X) storage reduction catalyst 4 a and the DPNR catalyst 4 b. This is achieved by increasing the catalyst temperature by continuously supplying fuel from the supplemental fuel valve 25, while controlling the air-fuel ratio of exhaust gas to the stoichiometric or richer ratio. The NO_(X) reduction control is intended to reduce the NO_(X) stored in the NO_(X) storage reduction catalyst 4 a and the DPNR catalyst 4 b to N₂, CO₂ and H₂O by intermittently supplying fuel from the supplemental fuel valve 25.

The PM catalyst regeneration control, the sulfur poisoning recovery control and the NO_(X) reduction control are performed individually as appropriate. When it is necessary to perform all three control simulataneously, these controls may be performed in the sequence described above.

Next, the correction process of fuel supply interval will be described. As stated previously, the volume of intake air to the engine 1 mounted on the vehicle decreases following certain environmental changes, such as atmospheric pressure change, or upon a shift from normal to transient driving. This increases the amount of PM emissions. As the amount of PM emissions increases, the amount of PM adhering and entering the injection hole of the supplemental fuel valve 25 increases, which helps PM deposits build up. The PM deposits may clog the injection hole of the supplemental fuel valve 25. As the exhaust gas temperature at the distal end of the supplemental fuel valve 25 increases from the reference preset temperature, the temperature of the distal end itself of the supplemental fuel valve 25 also increases, producing PM deposits. The PM deposits may clog the injection hole of the supplemental fuel valve 25.

In order to solve this problem, in this embodiment, a correction coefficient of the fuel supply interval, eminttemp, which is used for correcting the fuel supply interval, is calculated based on the temperature of the distal end of the supplemental fuel valve 25. In addition, a correction coefficient of the fuel supply interval, emintpm, which is used for correcting the fuel supply interval, is calculated based on the variation in amount of PM emissions due to environmental changes or during transient driving conditions. Between eminttemp and emintpm, the correction coefficient of the fuel supply interval that results in a larger amount of fuel supply per unit time, is selected as an target fuel supply interval. This provides the feature of maintaining fuel economy, while preventing clogging of the injection hole of the supplemental fuel valve 25.

A specific example of the correction process of fuel supply interval is described below with reference to the flowchart in FIG. 3. The ECU 100 executes the correction process of fuel supply interval. A routine of this correction process is repeated at a predetermined time interval.

In step ST1, the engine speed Ne is read from the output of the crankshaft position sensor 41 to calculate a required fuel supply amount Q based on the engine speed Ne with reference to a map, such as that shown in FIG. 4. The relationship between the engine speed Ne and the required fuel supply amount Q is obtained in advance by experiments, calculation, etc. Then, the map used to calculate the required fuel supply amount Q is prepared by plotting the relationship between the engine speed Ne and the required fuel supply amount Q, and stored in the ROM 102 of the ECU 100.

In step ST2, a reference fuel supply interval Tb (see FIG. 8) is calculated based on the required fuel supply amount Q and the engine speed Ne with reference to the map shown in FIG. 4. The map for calculating the reference fuel supply interval is also plotted with experimental and calculation data on the relationship between the required fuel supply amount Q and engine speed Ne, and the reference fuel supply interval Tb. The ROM 102 in the ECU 100 stores this map in advance. In step ST2, a reference exhaust gas temperature (ambient temperature of the supplemental fuel valve 25) is also obtained when the reference fuel supply interval Tb is calculated.

In step ST3, the correction coefficient of the fuel supply interval, eminttemp, which is used to correct the fuel supply interval, is calculated based on the temperature of the distal end of the supplemental fuel valve 25.

More specifically, based on the difference between the reference exhaust gas temperature obtained in step ST2 and the current exhaust gas temperature (change in exhaust gas temperature ΔTh), the correction coefficient of fuel supply interval, eminttemp, is calculated with reference to the map shown in FIG. 5. The map for calculating the correction coefficient of fuel supply interval shown in FIG. 5 is plotted with experiments and calculation data on the relationship between the variation in exhaust gas temperature ΔTh and the correction coefficient of fuel supply interval, eminttemp. The ROM 102 in the ECU 100 stores this map in advance. The correction coefficient of fuel supply interval, eminttemp, is preset smaller as the variation in exhaust gas temperature ΔTh increases. As the correction coefficient of fuel supply interval, eminttemp, which is calculated based on the temperature of the distal end of the supplemental fuel valve, is reduced, the fuel supply interval becomes shorter.

It should be understood that the exhaust gas temperature (ambient temperature of the supplemental fuel valve 25) may be calculated using a specific map for calculating the exhaust gas temperature. The map may use experimental and calculation data on engine speed Ne, intake temperature, atmospheric pressure and so forth as parameters. The ROM 102 in the ECU 100 may store this map in advance. Alternatively, an exhaust gas temperature sensor may be provided to detect and output the exhaust gas temperature upstream of the turbocharger 5.

In step ST4, the correction coefficient of fuel supply interval, emintpm, used to correct the fuel supply interval is calculated based the variation in amount of PM emissions due to environmental changes or during transient driving conditions.

More specifically, first an air volume ratio and a λ (excess air ratio) correction coefficient for amount of PM emissions are calculated.

The air volume ratio will now be described. The air volume ratio, gnr, is calculated by dividing the actual intake air volume to the engine 1, which is obtained from the output signal of the airflow meter 32, by the reference intake air volume on a flat driving condition (air volume ratio gnr=intake air volume divided by reference intake air volume).

Next the calculation of the λ correction coefficient for amount of PM emissions will be described. Based on the air volume ratio gnr calculated in the aforementioned process, and the atmospheric pressure (detected value), obtained from the output signal of the atmospheric pressure sensor 39, the λ correction coefficient, emgpmlmd, for amount of PM emissions is calculated with reference to a map of FIG. 6. The λ correction coefficient map of FIG. 6 is plotted with experimental and calculation data on the λ correction coefficient, using air volume ratio gnr and atmospheric as parameters. The ROM 102 in the ECU 100 stores this map in advance. The λ correction coefficient, emgpmlmd, is increased as the air volume ratio gnr and the atmospheric pressure decrease.

Based on the λ correction coefficient, emgpmlmd, thus calculated, the correction coefficient of fuel supply interval, emintpm, is calculated with reference to a map of FIG. 7. The map for calculating the correction coefficient of fuel supply interval shown in FIG. 7 is plotted with experiments and calculation data on the relationship between the k correction coefficient, emgpmlmd, and the correction coefficient of fuel supply interval, emintpm. The ROM 102 in the ECU 100 stores this map in advance. The correction coefficient of fuel supply interval, emintpm, is preset smaller as the variation in amount of PM emissions (λ correction coefficient, emgpmlmd) increases. As the correction coefficient of fuel supply interval, emintpm, decreases, the fuel supply interval becomes shorter.

In steps ST5 to ST7, the correction coefficient of fuel supply interval, eminttemp, calculated in step ST3, is compared with the correction coefficient of fuel supply interval, emintpm, calculated in step ST4. The smaller value of the two is selected, that is the one which results in a larger amount of fuel supply per unit time. More specifically, if the correction coefficient of fuel supply interval, eminttemp, which depends on the temperature of the distal end of the supplemental fuel valve, is smaller than the correction coefficient of fuel supply interval, emintpm, which depends on the variation in amount of PM emissions (if the result of the determination in step ST5 is true), the correction coefficient, eminttemp, is selected as a correction coefficient of the target fuel supply interval, emintad (step ST6). In contrast, if the correction coefficient of fuel supply interval, emintpm, is smaller than the correction coefficient of fuel supply interval, eminttemp (if the result of the determination in step ST5 is false), the correction coefficient, emintpm, is selected as a correction coefficient of the target fuel supply interval, emintad (step ST7).

In step ST8, the correction coefficient of the target fuel supply interval, emintad, selected in step ST6 or ST7, is multiplied by the reference fuel supply interval, calculated in step ST2, to obtain an target fuel supply interval (target fuel supply interval=[reference fuel supply interval prior to correction]×emintad). Then, the routine ends.

In accordance with the correction process of fuel supply interval, either the correction coefficient of fuel supply interval, eminttemp or emintpm, is selected to correct the target fuel supply interval. In particular, the correction coefficient that results in a shorter fuel supply interval (a larger amount of fuel supply per unit time) is selected. This allows the fuel supply interval to be appropriately corrected for either one of condition changes that is more likely to cause clogging of the injection hole of the supplemental fuel valve 25; where the temperature of the distal end of the supplemental fuel valve 25 rises or the amount of PM emissions increases due to environmental changes or during transient driving conditions. Thereby, clogging of the injection hole of the supplemental fuel valve 25 is effectively prevented. Also, in accordance with the correction process of fuel supply interval, a fuel supply amount (fuel supply amount per unit time) appropriate to the foregoing specific condition change is provided. This maintains fuel economy in contrast to the case where the fuel supply amount is adjusted when the amount of PM emissions reaches the maximum in the allowable fluctuation.

Although increasing the fuel supply amount per unit time prevents clogging of the injection hole of the supplemental fuel valve 25, the fuel also reacts with oxygen in the catalyst, which can cause the catalyst temperature to exceed a certain range of values (e.g. 750° C.). One approach to avoid such situation is offered as follows. The temperature of the DPNR catalyst 4 b is estimated based on the exhaust gas temperature detected by the exhaust gas temperature sensor 35. If the estimated catalyst temperature is equal to or higher than a prescribed temperature, the fuel supply amount per unit time is reduced according to the estimated catalyst temperature (more specifically, variation in catalyst temperature relative to a preset value). Therefore, an excessive rise in catalyst temperature is prevented. It should be understood that the prescribed temperature for the catalyst temperature may be obtained empirically by taking the certain range of the catalyst temperature (e.g. 750° C.) into consideration.

One approach to reducing the fuel supply amount per unit time, the fuel supply duration per interval shown in FIG. 8 may be shortened, while the target fuel supply interval remains unchanged after the fuel supply interval has been corrected. This approach not only ensures a shorter fuel supply interval, which prevents clogging of the injection hole of the supplemental fuel valve 25, also ensures a smaller total amount of fuel supply. Thus, while an excessive rise in catalyst temperature is prevented, clogging of the injection hole of the supplemental fuel valve 25 is also prevented.

To prevent an excessive increase in catalyst temperature, the following approach may also be taken. The temperature of the DPNR catalyst 4 b may be estimated based on the exhaust gas temperature detected by the exhaust gas temperature sensor 35. Then, based on the estimated current catalyst temperature and the target fuel supply interval the increase in catalyst temperature, resulting from the fuel supplies at the target fuel supply interval, is estimated. A restrictive correction or increase of the fuel supply amount per unit time is performed so that the estimated catalyst temperature does not exceed a prescribed value (a value determined based on the maximum allowable catalyst temperature).

Another embodiment of the invention is further described. In the embodiment described above, one of either the correction coefficient of fuel supply interval, eminttemp or emintpm, is used to determine the target fuel supply interval. However, the invention is not limited to the aforementioned embodiment. Instead, the target fuel supply interval may be calculated using only the correction coefficient of fuel supply interval, emintpm.

Also in the embodiment described above, the reference fuel supply interval Tb is multiplied by the correction coefficient of fuel supply interval, eminttemp or emintpm, to correct the fuel supply amount. Alternatively, the reference fuel supply duration per interval (see FIG. 8) may be multiplied by the correction coefficient of fuel supply interval, eminttemp or emintpm, to correct the fuel supply amount per unit time. To correct the reference fuel supply duration per interval, the correction coefficient of fuel supply interval, eminttemp, is preset larger as the variation in exhaust gas temperature ΔTh increases. In addition, the correction coefficient of fuel supply interval, emintpm, is preset larger as the variation in amount of PM emissions (λ correction coefficient, emgpmlmd) increases.

In the above-described embodiment, the correction coefficient of fuel supply interval, eminttemp, which depends on the temperature of the distal end of the supplemental fuel valve 25, is calculated based on the variation in exhaust gas temperature ΔTh. Alternatively, the correction coefficient of fuel supply interval, eminttemp, may be calculated based on the coolant temperature in the engine 1, obtained from a signal output by the water temperature sensor 31, with reference to a map of FIG. 9.

In the above-described embodiment, a direct-injection four-cylinder diesel engine is equipped with the exhaust gas purification system of the invention. However, the invention is not limited to this embodiment. Alternatively, other diesel engines having any number of cylinders, such as a direct-injection six-cylinder diesel engine, may be equipped with the exhaust gas purification system of the invention as well. In addition, the invention is limited to use with direct-injection diesel engines, but may also be applied to other types of diesel engines. Further, the invention may be used not only with vehicle engines, but also for engines designed for other purposes.

In the embodiment previously described, the catalytic converter 4 includes the NO_(X) storage reduction catalyst 4 a and the DPNR catalyst 4 b. Alternatively, the catalytic converter 4 may include a DPF in addition to the NO_(X) storage reduction catalyst 4 a or an oxidation catalyst.

While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the embodiments are shown in various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the invention. 

1. An exhaust gas purification system comprising: a catalyst disposed in an exhaust passage of an internal combustion engine; a supplemental fuel valve that supplies fuel to the exhaust passage; and an adjusting portion that adjusts a supplemental amount of fuel that is supplied from the supplemental fuel valve based on a variation in an amount of particulate matter emitted from a combustion chamber of the internal combustion engine.
 2. The exhaust gas purification system according to claim 1, wherein the adjusting portion adjusts the supplemental amount of fuel that is supplied from the supplemental fuel valve to the exhaust passage per unit time.
 3. The exhaust gas purification system according to claim 2, wherein the adjusting portion multiplies a reference fuel supply interval by a correction coefficient that depends on the variation in amount of particulate matter emitted to determine a target fuel supply interval.
 4. The exhaust gas purification system according to claim 3, wherein the adjusting portion multiplies the reference fuel supply interval by a correction coefficient to change the fuel supply interval to determine the target fuel supply interval, when an actual intake air volume to the internal combustion engine is smaller than a reference intake air volume.
 5. The exhaust gas purification system according to claim 1, wherein: the adjusting portion adjusts a fuel supply interval of fuel that is supplied from the supplemental fuel valve to the exhaust passage per unit time.
 6. The exhaust gas purification system according to claim 5, further comprising: a supplemental fuel valve temperature estimating portion for estimating a temperature of the supplemental fuel valve, wherein the adjusting portion compares a first correction coefficient with a second correction coefficient each other, the first correction coefficient is used to change the fuel supply interval when the actual intake air volume to the internal combustion engine is smaller than the reference intake air volume, the second correction coefficient is used to change the fuel supply interval when the estimated temperature of the supplemental fuel valve rises, and multiplies the reference fuel supply interval by the correction coefficient of the first and second correction coefficients that yields a shorter fuel supply interval to determine the target fuel supply interval.
 7. The exhaust gas purification system according to claim 1, further comprising: a catalyst temperature detecting portion for detecting a temperature of the catalyst, wherein the adjusting portion reduces the supplemental amount of fuel per unit time according to the detected catalyst temperature when the detected catalyst temperature is equal to or higher than a preset value.
 8. The exhaust gas purification system according to claim 1, further comprising: a catalyst temperature detecting portion for detecting a temperature of the catalyst, wherein the adjusting portion reduces the supplemental amount of fuel per unit time according to the detected catalyst temperature when the variation in the detected catalyst temperature is equal to or greater than a preset value.
 9. The exhaust gas purification system according to claim 1, further comprising: a coolant temperature detecting portion for detecting a coolant temperature in the internal combustion engine, wherein the adjusting portion reduces the supplemental amount of fuel per unit time according to the detected catalyst temperature when the variation in the detected coolant temperature is equal to or greater than a preset value.
 10. The exhaust gas purification system according to claim 7, wherein a fuel injection amount per unit time is reduced by shortening the fuel supply interval, while the fuel supply duration per interval is reduced.
 11. The exhaust gas purification system according to claim 1, wherein the internal combustion engine is a diesel engine.
 12. The exhaust gas purification system according to claim 1, wherein the internal combustion engine is mounted on a vehicle.
 13. The exhaust gas purification method comprising: supplying fuel to an exhaust passage of an internal combustion engine, the exhaust passage having a catalyst disposed therein; and adjusting an supplemental amount of fuel that is supplied from the supplemental fuel valve based on a variation in an amount of particulate matter emitted from a combustion chamber of the internal combustion engine.
 14. The exhaust gas purification method according to claim 13, wherein the supplemental amount of fuel that is supplied from the supplemental fuel valve to the exhaust passage per unit time is adjusted in order to adjust the supplemental amount of the fuel.
 15. The exhaust gas purification method according to claim 14, wherein a reference fuel supply interval is multiplied by a correction coefficient that depends on the variation in amount of particulate matter emitted to determine a target fuel supply interval in order to adjust the supplemental amount of the fuel.
 16. The exhaust gas purification method according to claim 15, wherein the reference fuel supply interval is multiplied by a correction coefficient that adjusts the fuel supply interval to determine the target fuel supply interval when an actual intake air volume to the internal combustion engine is smaller than a reference intake air volume in order to adjust the amount of the fuel.
 17. The exhaust gas purification method according to claim 13, wherein the degree of fuel is an interval of fuel that is supplied from the supplemental fuel valve to the exhaust passage.
 18. The exhaust gas purification method according to claim 17, further comprising: estimating a temperature of the supplemental fuel valve, wherein a first correction coefficient and a second correction coefficient are compared with each other, the first correction coefficient is used to change the fuel supply interval when the actual intake air volume to the internal combustion engine is smaller than the reference intake air volume, and the second correction coefficient is used to change the fuel supply interval when the estimated temperature of the supplemental fuel valve rises; and wherein the reference fuel supply interval is multiplied by the correction coefficient of the first and second correction coefficients that yields a shorter fuel supply interval, whereby the target fuel supply interval is determined.
 19. The exhaust gas purification method according to claim 13, further comprising: detecting the catalyst temperature, wherein the supplemental amount of the fuel per unit time is reduced when the detected catalyst temperature is equal to or higher than a preset value in order to adjust the supplemental amount of the fuel.
 20. The exhaust gas purification method according to claim 19, wherein a fuel injection amount per unit time is reduced by shortening the fuel supply interval, while the fuel supply duration per interval is reduced. 